WASHINGTON—The White House issued its strongest denunciation yet of Russian President Vladimir Putin over the downing of the Malaysia Airlines flight in eastern Ukraine, saying Friday that available evidence shows the Russians are "culpable" in the plane crash.

The charge came on the same day Pentagon officials said Moscow was preparing to transfer sophisticated rocket systems to pro-Russia separatists in Ukraine. U.S. officials said that while Russia has previously sent ground-attack rockets across the border,...